1. Field of the Invention
The present invention relates to an image data communication system, and more particularly, to a method of inhibiting error propagation in a bidirectional digital compressed image data communication system. The present application is based on Korean Application No. 00-4050, which is incorporated herein by reference.
2. Description of the Related Art
FIG. 1A is a schematic diagram illustrating one way of configuring a bidirectional digital image data communication system. In FIG. 1A, two nodes 100a and 150a, which are connected to each other via a communication network, include encoders 102a and 152a, respectively, and decoders 104a and 154a, respectively, and communicate with each other via a bidirectional communication channel.
FIG. 1B is a schematic diagram illustrating another way of configuring a bidirectional digital image data communication system. In FIG. 1B, a node 100b includes only an encoder, and another node 150b includes only a decoder. In this case, image data is only transmitted from the node 100b having an encoder to the node 150b having a decoder via a forward channel. A backward channel is used to transmit feedback information on the image data received via the forward channel. Of course, a bidirectional digital image data communication system can be configured by the combination of FIGS. 1A and 1B.
In an image data communication system, image data undergoes data compression and decompression to effectively use the bandwidth of a communication channel. That is, an external video signal is compressed during encoding by the encoder 102a, 152a, or 100b and transmitted via a communication network. The decoder 104a, 154a, or 150b decompresses the received compressed image data to restore the original image data during decoding, and outputs a restored video signal.
FIG. 2A is a schematic diagram illustrating a way of encoding each image frame in a digital image data communication system.
The way of encoding an image frame includes intracoding and intercoding. Intracoding is encoding an image frame using the correlation between pixels within a screen, and intercoding is encoding a current image frame with reference to the previous frame (or the next frame) using the correlation between screens. An image frame encoded by intracoding is referred to as an I frame, and an image frame encoded by intercoding is referred to as a P frame. When an image frame is encoded with reference to the next image frame, it is referred to as a B frame. Hereinafter, the term P frame is used to refer to any interceded frame, including a B frame.
Intercoding generally has a high compression efficiency, so that most image frames are encoded by intercoding using the correlation between frames. That is, as shown in FIG. 2A, the first image frame 200a in a sequence is encoded by intracoding to constitute an I frame, and the subsequent image frames 210a, 220a, 230a and 240a are encoded by intercoding to constitute P frames, until the corresponding sequence is ended.
However, this intercoding method using the correlation between frames has a problem of error propagation. FIG. 2B is a schematic diagram illustrating a conventional error propagation mechanism in a digital image data communication system.
As shown in FIG. 2B, it is assumed that the first and second frames 200b and 210b in a sequence have no errors, while the third frame 220b has an error 222 at a specific block. If the intercoding method is used, the fourth frame 230b is encoded with reference to the third frame 220b, so that the error 222 in the third frame 220b is propagated into an error 232 in the fourth frame 230b. Similarly, the fifth frame 240b has an error 242 propagated from the error 232 of the fourth frame 230b. 
That is, in the intercoding method, if an unrestorable error is generated at a specific block on a frame during transmission or for other reasons, it is not limited to the corresponding frame but affects the subsequent frames. Also, this propagated error is gradually magnified, since each block in a current frame is encoded with reference to both a corresponding block in the previous frame and its adjacent blocks in the intercoding method.
Various mechanisms have been designed for inhibiting the error propagation caused in the intercoding method. FIG. 2C is a schematic diagram illustrating a conventional error propagation prevention mechanism in a digital image data communication system.
In FIG. 2C, the I frame 200c is obtained by intracoding, and specific blocks 214, 224, 234, 244 of P frames 210c, 220c, 230c and 240c, respectively, obtained by intercoding, are compulsorily encoded by intracoding. A current recommendation is that an I block (which is a block compressed by intracoding) be included at least every 132 image frames, but a detailed method of selecting the I block is not prescribed separately.
In the intercoding method, encoding is performed using the correlation between frames, so that both a previous frame and a current frame are referred to during encoding, and a process such as discrete cosine transformation (DCT) for encoding includes a floating point operation. Of course, decoding also includes a similar floating point operation. However, the results of the floating point operations of an encoder and a decoder may have a slight difference depending on the type of the corresponding system. Thus, in order to prevent accumulation of this slight difference, inclusion of one I block at least every 132 image frames has been recommended.
The technique of compulsorily intercoding a specific block as shown in FIG. 2C prevents the propagation of an error which is caused by the difference between the result of floating point operation of an encoder and that of a decoder. However, this technique cannot fundamentally prevent propagation of an unrestorable error that occurs during transmission or for other reasons.
Of course, a decoder minimizes an error recognized by a person while visually watching images, using a technique such as concealment, when an unrestorable error is detected from a specific block on a received image frame. However, this technique cannot fundamentally correct an error, so the conventional intercoding method cannot prevent an error from being propagated from a frame to another frame.